I realise a lot has already been said about this post, but I think I have a point that hasn't been brought up yet. As a visual arts grad, I spent a lot of my first year learning colour theory, and I am going to attempt to explain optics. If there is an optometrist on EN world, I'm sorry if I've made any mistakes.
Okay, now that I've finished with my disclaimer . . . The reason why we see things is solely based on light. Right now, I can distinguish between the computer and the wall behind it because light is reflecting into my eyes differently. The frequency of the light wave tells me what hue it is, and what shade it is.
The Darkvision ability states that you can see in black and white, which I interpret to mean SHADES. Our eyes contain cones and rods. Cones tell us about hues, our rods tell us about shades. If I was completely colour-blind, my cones would not operate, and I would see the world as a black and white film. I would not interpret darkvision as having no contrast, because of the problems with this that other people have already pointed out (ie. not being able to see the cave opening, because the stone is only slightly lighter than the shadows within). I would not consider the echolocation theory for one reason: Darkvision does not give you any bonuses when detecting someone hidden by Greater Invisibility. If it were a matter of echolocation, then you would instantly know that the invisible wizard was standing 3 metres infront of you. Besides, echolocation for eyes is what our vision is!
Now, Darkvision states that the person can see WITHOUT light source. There are only TWO possible explainations for this. 1) Darkvision is magical. End of story. Or, more rationally, 2) Darkvision operates as a VERY lowlight vision.
Consider, the only place where there is absolutely NO light is a blackhole. Does your campaign take place within a black hole? If so, you've got a whole host of other problems. Otherwise, there has to be SOME light, just not very much at all. Our eyes would not be able to detect it. If you treat Darkvision as a VERY lowlight vision, all your problems are solved:
Of course the drow in the "lightless" cavern can read, because some miniscule amount of light is bouncing off the white paper, and off the black ink (at a very different frequencies) and into his eyes. He can also see a reflection in the mirror, because some small amount of light is bouncing off the cavern walls, into the mirror, and then back into his eyes. When he looks at a pane of glass, he sees the next room, which is slightly lighter shades than the room he is in, due to refraction through the glass.
Whew. I know this is very long, but it was the shortest way I could express myself. Please keep in mind, that no matter how black you may think a cavern in the underdark is, physics dictates that there IS some amount of light, it just is not perceived by the limited visual range of humans.
R from Three Haligonians